Press-hardening or PHS steel sheets appeared in the final years of the 20th century; from these, parts with extremely high strength can be produced by the steel sheet being heated above the austenitization temperature and through cooling down during the pressing, a substantially purely martensitic structure is obtained. It is customary to provide these steel sheets with a zinc coating as corrosion protection. Such a layer can be obtained electrogalvanically or through dipping in a zinc melt. The melting temperature of pure zinc is 420° C. and is thus low enough so as not to attack the steel during the dipping in the zinc bath and not change its crystal structure. For this reason, a corrosion protection layer on large surfaces can be quickly and cost-effectively created by dipping or hot dip galvanizing.
The boiling temperature of zinc is around 906° C. When the steel sheet is heated above the austenitization temperature, i.e. above 900° C., there is therefore the danger that the zinc layer evaporates. DE 20 2004 021 264 U1 proposes mixing the zinc layer with a small amount of an oxygen-affine element, which on the surface of the zinc layer forms an oxide skin and because of this is to prevent the evaporating of the zinc. Among the elements magnesium, silicon, titanium, calcium, aluminum, boron and manganese recommended for this, silicon, aluminum and nickel are known for forming an air-tight passivation layer on their surface, which protects underlying metal from oxidation. The exemplary embodiments described in DE 20 2004 021 264 U1 use exclusively aluminum as oxygen-affine element.
Even if through the addition of the oxygen-affine element a tight oxygen skin is actually obtained on the surface of the zinc layer, this is not able to remedy another problem that occurs during the processing of zinc-coated press-hardening metal sheets, namely the so-called liquid metal corrosion. The term stands for an entering of the liquid zinc along grain boundaries of the steel, which leads to an embrittlement of the formed parts produced from the metal sheet.
DE 20 2004 021 264 U1 also mentions the technology of “galvannealing”, in which a hot dip galvanized metal sheet is heated above the melt temperature of the zinc in order to convert the zinc layer into a zinc-iron alloy layer—which is likewise effective as corrosion protection—through diffusion. Here, too, there is the danger of liquid metal corrosion.
In order to avoid embrittlement through liquid metal corrosion it has been necessary up to now to heat the steel sheet to the austenitization temperature so slowly that the surface layer of zinc can form a zinc-iron alloy layer without a molten phase occurring. The necessity of heating the steel sheet slowly leads to long cycle times and substantially impairs the productivity of the parts production from such a steel sheet.
In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.